1. Field of the Invention
The present invention is related to a Universal Serial Bus (USB) apparatus and a method thereof, and more particularly, to a Universal Serial Bus (USB) apparatus without an accurate frequency oscillator and method thereof.
2. Description of the Prior Art
Universal Serial Bus (USB) is a common serial transmitting interface utilized in computer systems. It is very convenient for USB to establish connections in a high transmission speed (e.g., 480 Mbps of USB 2.0). Therefore, USB is becoming a very popular transmitting interface, and almost every apparatus available on the market today is compatible with the USB protocol to provide a connecting interface with computer systems, such as portable card readers, portable storage apparatus, and scanners.
Normally, the transmitting frequency of the USB interface is required to have an accurate operating frequency. When operated at low-speed or full-speed (LS/FS) mode, the frequency error must be less than +/−2500 ppm, and when in high-speed (HS) mode, the frequency error must be less than +/−500 ppm. Therefore, an accurate oscillating signal source external to the USB chip is utilized to provide an accurate frequency to the USB chip to achieve the requirement of the frequency error. FIG. 1 illustrates a USB system 10 including a USB host 12, a USB apparatus 14 and an accurate oscillating signal source 16 such as a crystal oscillating component. A special oscillating apparatus can also be utilized to obtain the desired frequency accuracy.
However, the cost of the above-mentioned special oscillating apparatus is very high. Therefore, in the low-cost application fields, to develop a USB chip that does not require the accurate external oscillating signal source is very important.